Broken link to md_flexible example

In the [home page of AutoPAS](https://autopas.github.io/doxygen_documentation/git-master/), the link to the md-flexible example is broken: https://autopas.github.io/doxygen_documentation/git-master/examples/md-flexible

I would have fixed it already, but I don't really understand where this page draws its content from.

Start by inspecting the AutoPas.github.io repository and the linked AutoPAS home page to locate the source that generates the page content. Trace the md-flexible example link and update its source so the link resolves successfully at the documented URL.

Written by the indexing model from the issue text.